Display correction waveform generator for multiple scanning frenquencies

ABSTRACT

A method for generating display correction waveforms for a CRT display comprises the steps of selecting one of a plurality of trace portions for forming part of a correction waveform, the trace portions having different average values. Completing each of the correction waveform by combining each selected trace portion with a respective retrace portion such that all completed correction waveforms have a predetermined average value. The correction waveforms may have vertical and/or horizontal rates.

[0001] This invention relates generally to correction waveformgeneration in an image display, and more particularly to a waveformgenerator operable at a plurality of display standards.

[0002] Waveforms comprising multiple frequency components frequentlyinclude a DC component which renders the signal essentially unipolar.However, removal of the DC component, for example, by capacitivecoupling results in the loss of the unipolar characteristic with theresultant waveform being disposed equally in area, positively andnegatively about an average waveform value. This average value isdependent on the waveform shape, thus when AC coupled waveforms withdiffering shapes produce differing average values with respect to thewaveform peaks. Hence the AC peak potentials received by the circuitryfollowing the AC coupling are altered, and vary in accordance withdiffering waveform shapes.

[0003] In an exemplary cathode ray tube the distance from the center ofelectron beam deflection is, in general, shortest to the center of thedisplay screen, with the distance increasing to maximum values in thescreen corners. Thus to achieve consistent beam landing or a focusedelectron beam over the complete screen area requires that a DC focusvoltage is combined with a signal waveform comprising multiplefrequencies, for example, horizontal and/or vertical frequency parabolicshaped waveforms. Typically this parabolic waveform is generated withlow voltages near the system ground potential and added to the highvoltage DC focus voltage via an AC coupling. The amplitude of thisparabolic signal has a factory determined value, since the distancesbetween all screen locations and the center of electron beam deflectionare known and fixed. Thus, a single focus control, which adjusts a DCpotential may be provided to allow optimum focus to be obtained, notonly in the screen center, but at all screen locations. Such overalloptimized adjustment assumes an accurately determined factory setamplitude value for the generally parabolic shaped signals.

[0004] Although the geometric relationship between the display screenand electron beam are fixed and hence not standards specific, a displaydevice may be capable of operation at multiple display standards withvarious scanning frequencies and differing retrace and blanking times.Thus a parabolic waveform generator is required which is responsive tothe display standard, follows the scanning frequency, is capable ofdiffering phasing relative to a vertical retrace pulse and is responsiveto differing blanking durations. Such varieties of waveform shaping andphasing consequently vary the AC peaks with respect to the DC componentof the waveform. Hence, when this exemplary waveform is ultimately ACcoupled for addition to the high DC voltage for DC focus control, theloss of the DC component of the waveform may necessitate readjustment oroptimization of the DC focus control voltage. Hence a display operableat multiple scanning and display standards, may require individual focuscontrol adjustment for each display standard.

[0005] U.S. Pat. No. 5,471,121 discloses a circuit for generating adynamic focus voltage. The focus voltage is compared to a referencevoltage. Whenever the focus voltage drops below the reference voltage, aswitching signal is provided. The switching signal controls a switch,which provides a DC focus voltage during the overscan portions of thetrace period. The DC focus voltage has such a value to keep the averagevalue of the focus voltage essentially constant.

[0006] In EP-A 0 554 836 a parabolic waveform generating apparatus isdescribed. The known apparatus comprises two memory devices, eachstoring data necessary for generating parabolic waveforms necessary forthe images of a specific aspect ratio. For different aspect ratios anarithmetic logic unit processes the output data of the two memorydevices to generate new output data values for a desired parabolicwaveform.

[0007] DE-A 197 54 904 provides a method for generating a focus voltagefor a CRT. The known method comprises the steps of verifying ifreference data have changed, calculating correction data and generatinga focus voltage according to the correction data.

[0008] It is an object of the preset invention to provide a method forgenerating display correction waveforms for a CRT display. This objectis solved by a method according to claim 1. The inventive methodcomprises the steps of selecting one of a plurality of trace portionsfor forming part of a correction waveform, the trace portions havingdifferent average values. Completing each of the correction waveform bycombining each selected trace portion with a respective retrace portionsuch that all completed correction waveforms

[0009] An exemplary digital parabolic waveform signal generator isillustrated in FIG. 1, coupled, for example, to provide dynamic focus,or electron beam landing correction in a cathode ray tube. The digitalparabolic waveform signal generator is depicted in area 100, which mayform part of an inventive integrated circuit, for example, STMicroelectronics type STV2050. The digital waveform generator 100 iscoupled to area 200 which includes a differential amplifier and lowpassfilter, which is coupled to dynamic focus signal generator 250, andcathode ray tube CRT.

[0010] Digital waveform generator 100 includes a digital controller CTRL105 which controls all functions within generator 100 via data bus 115,and provides control communication via a data bus 70 to an externalmicroprocessor 75. A RAM 110 is connected to an external EEPROM memory(PROM 80) via a dedicated data bus 85 from which it receives operatingdata at power on. RAM 110 stores operating data values for an exemplarydisplay device, but in particular, data specific to the generation of aparabolic waveform shape. To achieve good results for the display atdifferent scanning frequencies and display standards, the focus voltageis modulated both in horizontal and vertical direction with regard tothe beam landing position on the screen. The focus voltage is modulatedby parabolic waveforms wherein the waveforms for vertical and horizontalmodulation, respectively, are generated separately. Of course it is alsopossible to modulate the focus voltage only by either a vertical or ahorizontal waveform.

[0011] At first the vertical parabolic waveform is considered. Data bus115 supplies vertical parabola specific data from RAM 110 to verticalparabola generator 120, vertical coefficient data to generator 130 andvertical compensation data to generator 140. Parabola generator 120generates a vertical parabolic waveshape Vpar represented by six bitdigital values in accordance with specified amplitude values orcoefficients, occurring at specific times during trace or active picturetime. Compensation generator 140 forms a six bit digital value Vcompwhich is coupled as one input to selector switch 150. Output Vpar fromgenerator 120 is coupled as a second input to switch 150 which iscontrolled by a vertical rate signal Svrt occurring during a verticalretrace period. Thus switch 150 couples the digital parabolic waveformVpar to digital to analog converter 160 during active picture, orvertical trace time, and selects digital word Vcomp for digital toanalog conversion by DAC 160 during the vertical retrace period. Digitalto analog converter 160 generates an analog signal which is coupled to afirst input of differential amplifier 170.

[0012] In a similar way the horizontal parabolic waveform is generated.Data bus 115 also supplies horizontal parabola specific data from RAM110 to horizontal parabola generator 121, horizontal coefficient data togenerator 131 and horizontal compensation data to generator 141.Horizontal parabola generator 121 generates a parabolic waveshape Hparrepresented by six bit digital values in accordance with specifiedamplitude values or coefficients, occurring at specific times duringhorizontal trace or line period. Compensation generator 141 forms a sixbit digital value Hcomp which is coupled as one input to selector switch151. Output Hpar from generator 121 is coupled as a second input toswitch 151 which is controlled by a horizontal rate signal Shrtoccurring during a horizontal retrace period. Thus switch 151 couplesthe digital parabolic waveform Hpar to digital to analog converter 161during an active line, or horizontal trace time, and selects digitalword Hcomp for digital to analog conversion by DAC 161 during thehorizontal retrace period. Digital to analog converter 161 generates ananalog signal as a differential output which is coupled to a secondinput of differential amplifier 170.

[0013] Amplifier 170 is configured as a differential input amplifier,with input resistors R1 and R2 of similar values to provide improvedstability with temperature. However, it is also possible to manipulatethe summing of the input signals by selecting different values for theresistors R1 and R2. Then the input parabolic signals are summed ininverse proportion to the value of the input resistors R1 and R2. Inthis way an individual weighting coefficient can be assigned to each ofthe inputs of amplifier 170. The gain of amplifier 170 is determined inpart by resistor R2 and R3 and capacitor C1, which provides frequencydependent negative feedback. The analog signal from DAC 160 supplied tothe first input of the amplifier 170 is parabolic in shape comprised ofup to 64 discrete amplitude levels where each level, or amplitude valueis held constant for a number of line periods. These discrete amplitudevalues, which describe the vertical parabola, are only permitted tochange during horizontal retrace periods.

[0014] The analog signal from DAC 161 is parabolic in shape as well witha resolution of 64 discrete amplitude levels. The amplitude valueschange during horizontal trace e.g. at positions corresponding to thehorizontal positions of vertical lines of a grid which is used duringthe setting of convergence correction values. It is possible to adaptthe timing of the changes of the focus voltage for horizontal modulationaccording to the requirements of different modes of operation, forexample zoom modes.

[0015] The changes in parabolic signal values or steps generatetransients which are removed by lowpass filtering resulting from feedback capacitor C1 of amplifier 170 and lowpass filtering at theamplifier output provided by series connected resistor R4 and shuntconnected capacitor C2.

[0016] The lowpass filtered, vertical rate parabolic signal Vpar,depicted in FIG. 2A, is coupled via resistor R5 to amplifier 180 of area250. As is well known, negative feed back from the amplifier output viaresistor R7 forms a low, or virtual earth input impedance. Amplifier 180also provides voltage gain such that the output signal has an amplitudein the range of approximately 600 V which is coupled via capacitor C3 tothe wiper of focus potentiometer Rf. Thus the summed vertical andhorizontal rate parabolic signals form a focus modulation signal Fmwhich is added to the DC focus voltage Vf, for example 8.5 kV, generatedby potentiometer Rf and applied as waveform Vfm to the focus electrodeof cathode ray tube CRT.

[0017] Coefficient generator 130 forms vertical parabola amplitudedetermining coefficients as three digital words V1, V2 and V3 which setthe amplitude of the parabola at specific time intervals to be generatedby generator 120. The coefficients are independent of one another buthave fixed positions or line counts relative to each other within theperiod of a field. For example, in FIG. 2A, the time between ordinatesV1 and V2 is the same as that between ordinates V2 and V3. A fieldrepetition rate parabola is illustrated in FIG. 2A, with a maximumamplitude defined by 6 bits, giving 64 possible amplitude values. Theparabola position or phase within the field period is also adjustable,for example by offsetting a starting point of a counter which determinesthe time between ordinates V1, V2 and V3. The vertical positionadjustment of phasing of the parabolic waveform may be performed by anexemplary remote control RC73 which communicates with microprocessor 75via an infra red receiver IRRX, 72, or during factory setup by a directdata bus connection to microprocessor 75 (not shown).

[0018] Generator 120 performs calculations which cause the generation ofa parabola that passes through the three user defined amplitude values.The general form of equation for a parabolic waveform generation is,

Parabola=ax ² +bx+c,   (1)

[0019] where variables a, b, c and Z are calculated as follows from theuser defined values for V1, V2 and V3,

a=1/Z ²*(2V3−4*V2+*V1),   (2)

b=1/Z*(−V3+4*V2−3*V1),   (3)

c=V1,   (4)

Z=12*(VGD+1),   (5)

[0020] where VGD, vertical grid distance, represents a vertical imagedimension measured in scan lines, which may have values between 11 and63. During setup the parabola amplitude coefficients V1, V2 and V3 areadjusted in conjunction with focus control Rf to achieve optimum overallCRT focus.

[0021] Data representing coefficient V4 is read from RAM 115 and formedinto digital word V4 by compensation data generator 140. Data switch 150provides selection between parabola data from generator 130 andcompensation data representative of a fixed or DC value from generator140. Switch 150 is controlled by a vertical rate signal Svrt to selectDC compensation data during the vertical retrace period and parabolicwaveform data for the active part of the field period. The functioncoefficient V4 value will be explained with reference to FIGS. 2A, 2Band 2C.

[0022] Coefficient generator 131 forms horizontal parabola amplitudedetermining coefficients as three digital words H1, H2 and H3 which setthe amplitude of the horizontal parabola at specific time intervals tobe generated by generator 121. The coefficients are independent of oneanother but have fixed positions relative to each other within theperiod of a line. For example, the time between ordinates H1 and H2 isthe same as that between ordinates H2 and H3. The parabola position orphase within the line period is also adjustable, for example byoffsetting a starting point of a counter in the same way as explainedfor the vertical parabola. The horizontal parabola itself is calculatedin a similar way as the vertical parabola by replacing V1, V2, and V3with H1, H2, and H3, respectively in equations (1) to (4) given above.The parameter Z is selected to be

Z=14   (5′).

[0023] However, it is also possible to select a different value and theinvention is not limited to a specific value. It is preferred to utilizevalues that are easily deductible from the horizontal timing that isrequired to synchronise the focus voltage modulation with the deflectionof the electron beam.

[0024] Data representing coefficient H4 is read from RAM 115 and formedinto digital word H4 by compensation data generator 141. Data switch 151provides selection between parabola data from generator 131 andcompensation data representative of a fixed or DC value from generator141. Switch 151 is controlled by a horizontal rate signal Shrt to selectDC compensation data during the horizontal retrace period and parabolicwaveform data for the active part of the line. The function ofcoefficient H4 is similar to that of coefficient V4. The explanationwill be given below with reference to FIGS. 2A, 2B and 2C.

[0025] In FIG. 2A parabolic signal Vpar is illustrated with coefficientV4 having two different values namely V4 and V4′ (shown with a dashedline). FIGS. 2B and 2C depict signal Vpar, of FIG. 2A, coupled viaamplifier 180 and capacitor C3 to form of focus waveform Vfm. However,since the horizontal component of waveform Vfm is approximately doublethat of the vertical component, in the interest of drawing clarity FIGS.2B and 2C show only the vertical rate parabolic component of signal Fm.

[0026] The AC coupling of signal Fm by capacitor C3 results in the lossof the waveform DC component, which consequently causes signal Fm to bedisposed symmetrically, in terms of waveform polarities, about the DCfocus voltage Vf. Thus, as described, with the amplitude of signal Fmbeing factory determined and preset, exemplary focus control Rf may beadjusted to achieve optimum CRT focus at the screen center by means ofpeak voltage value Vfc, with focusing at the screen top and bottom beingdetermined by cusp voltages Vft and Vfb respectively. In actuality, ifwaveform Vfm is appropriately shaped by means of horizontal and/orvertical coefficient value manipulation, optimum focus may be achievedover the whole CRT display surface.

[0027] However, as has been described previously, changes in thevertical parabolic signal shape, for example as depicted in FIG. 2A bycoefficient V4′ shown with the dashed line, cause the mean value to bedifferent. In FIGS. 2B and 2C the parabolic waveforms Vparab areidentical in both shape and amplitude. For example, in FIG. 2B if thewaveform amplitude is measured relative to the mean or average value bythe addition of values Vft+Vfc, this value is equal to the correspondingsignal amplitudes Vft′+Vfc′ of FIG. 2C. However, since the mean valuesof the waveforms shown in FIGS. 2B and 2C are different, the optimizedcenter screen focus of FIG. 2B, resulting from the addition of exemplarypeak signal amplitude Vfc and DC value Vf is no longer optimum for thewaveform depicted in FIG. 2C as a consequence of the diminished peakamplitude of signal Vfc′ relative to the mean value of the waveform. Infact the whole screen is defocused as a result of the differing meanvalues which necessitates readjustment of focus control Rf to restoreoverall optimum focus.

[0028]FIGS. 2B and 2C illustrate that coefficient V4, which is selectedduring the vertical retrace period and thus plays no CRT electrodecontrol, may advantageously provide compensation for changes in the meanvalue of focus modulation waveforms generated for differing display ordeflection standards. For example, differing display standards may beconsidered with reference to FIG. 2A, which indicates a field periodcomprising a vertical retrace or vertical blanking interval Tvrt andactive scan period 2T. In the NTSC television signal format, the fieldperiod comprises 262.5 horizontal line periods with interval Tvrtrepresenting approximately 20 line periods, thus the ratio of retrace orvertical blanking interval to the field period is approximately 1:13 or8%. However, in the ATSC 1080I high definition television standard orANSI/SMPTE standard 274M, a frame comprises 1125 lines with 1080 activeline periods. Thus there are 45 lines of non-active picture per framewhich in the interlaced format would be distributed between each fieldcomprising 562.5 horizontal line periods. The non-active picture orblanking and vertical retrace interval Tvrt representing approximately22.5 line periods. Hence the ratio of retrace or vertical blankinginterval to the field period is approximately 1:25 or 4% which isapproximately half that of the NTSC format. This ratiometric differencein waveform shape or timing may be obviated by advantageous use ofcoefficient V4 which has differing preset standard specific values,selected to maintain optimum beam landing or focus by compensating fordifferences in the average value of vertical rate correction waveforms.

[0029]FIG. 3 depicts a parabolic waveform shape, for example, generatedin accordance with a display image having a vertical blanking widthdifferent to that of the signal for which the parabolic signal of FIG.2A was generated. The waveform depicted in FIG. 3 is shaped inaccordance with the values of ordinates V12, V22 and V32, where ordinateV12 is delayed or phase shifted by TΦ relative to the start of thevertical retrace interval Tvrt. In addition the waveform shape may beconsidered to represent a parabola superimposed on a field rate ramp orsawtooth signal as depicted by broken line S. Advantageouslycompensation data word V42 provides an adjustable signal component whichallows differing waveform shapes to have substantially similar DCcomponents, thus facilitating operation at multiple display standardswithout focus readjustment or multiple focus values.

[0030] The same explanation is applicable to the coefficient H4 which isselected to maintain optimum beam landing or focus by compensating fordifferences in the average value of horizontal rate correctionwaveforms.

[0031]FIG. 4 shows an alternative embodiment of the present invention.The main difference compared to the embodiment in FIG. 1 is that thevalues of the horizontal and vertical parabolas Hpar, Vpar and thecompensation coefficients H4 and V4 are added at first in adder stage162. Subsequently, the sum value is converted into an analog value byDAC 160 and supplied to the first input of differential amplifier 170.The second input of amplifier 170 is connected to a fixed referencepotential Uref. The output of amplifier 170 is processed in the same wayas described for the previous embodiment. Evidently the embodiment shownin FIG. 4 yields the same effects and advantages as set out aboverequiring only a single digital to analog converter.

1. A method for generating display correction waveforms for a CRTdisplay, comprising the step of selecting one of a plurality ofpredetermined trace waveform portions extending during the trace period,generated by a waveform generator and forming part of a correctionwaveform, the selectable trace waveform portions having differentaverage values characterized by the step of completing the correctionwaveform by combining the selected trace waveform portion with arespective retrace waveform portion extending during the retrace periodsuch that the completed correction waveform has a predetermined averagevalue independent of the selected trace waveform portion.
 2. The methodof claim 1, further comprising the step of: selecting among saidplurality of trace waveform portions in accordance with differentoperating characteristics of said CRT display.
 3. The method of claim 2,wherein said different operating characteristics comprise a plurality ofdisplay scanning standards.
 4. The method of claim 1, further comprisingthe step of: determining coefficient values to form said plurality oftrace waveform portions; and, storing said plurality of trace waveformportions.
 5. The method of claim 1, further comprising the step of:determining coefficient values to form said plurality of retracewaveform portions; and, storing said plurality of retrace waveformportions.
 6. The method of claim 1, further comprising the step ofsuperimposing correction waveforms having a vertical rate withcorrection waveforms having a horizontal rate.
 7. Method of claim 6,comprising the step of adding digital values representing saidcorrection waveforms having vertical and horizontal rates, respectively.8. Method of claim 6, comprising the step of assigning a weightingcoefficient to each of said correcting waveforms having vertical andhorizontal rates, before, superposition of the waveforms.
 9. Anapparatus for generating display correction waveforms for a CRT display,comprising means (120, 121) for selectively generating one of aplurality of predetermined trace waveform portions extending during thetrace period for forming part of a correction waveform, the selectabletrace waveform portions having different average values characterized bymeans (150, 151) for combining the selected trace waveform portion tocomplete the correction waveform such that the completed correctionwaveform has a predetermined average value independent of the selectedtrace waveform portion.
 10. The apparatus of claim 9, wherein saidgenerating means (120, 121) is responsive to a group of coefficientvalues (V1, V2, V3; H1, H2, H3).
 11. The apparatus of claim 10, whereinsaid group of coefficient values (V1, V2, V3; H1, H2, H3) are coupled tosaid generating means (120, 121) in accordance with an operating mode ofsaid CRT display.
 12. The apparatus of claim 9, wherein said respectiveretrace waveform portion is generated responsive to a coefficient value(H4, V4).
 13. The apparatus of claim 9, wherein said combining means(150, 151) is controlled responsive to a retrace signal (Svrt, Shrt) andselects said retrace waveform portion during a period of said retracesignal with said trace waveform portion selected during an absence ofsaid retrace signal.
 14. The apparatus of claim 9, further comprisingmeans (170, 171) for superimposing correction waveforms having avertical rate with correction waveforms having a horizontal rate. 15.The apparatus of claim 14, comprising means (R1, R2) for assigning aweighting coefficient to each of said correcting waveforms havingvertical and horizontal rates before superposition of the waveforms. 16.The apparatus of claim 14, comprising means (162) for adding digitalvalues representing said correction waveforms having vertical andhorizontal rates, respectively.
 17. The apparatus of claim 9, furthercomprising a capacitor (C3) for AC coupling said correction waveform tosaid CRT display for correction wherein said predetermined average valuepermits said AC coupling of said correction waveform without anysubstantial change in said predetermined average value.
 18. Theapparatus of claim 9, further comprising a capacitor for AC couplingsaid correction waveform to said CRT display for correction, saidcorrection waveform having a peak value relative to said predeterminedaverage value, wherein said predetermined average permits said AC,coupling of said correction waveform without any substantial change insaid peak value relative to said predetermined average value.
 19. Theapparatus of claim 9, further comprising a capacitor for AC couplingsaid correction waveform to said CRT display for correction of electronbeam landing errors.